1. Field of the Invention
The present invention relates to the amplification of audio signals. It finds applications in all audio appliances, in particular in mobile phones. In such an application, an audio amplification device is designed to drive the loudspeaker or the headphone socket of a mobile phone.
2. Description of the Related Art
Typically, an audio amplifier is biased in the mid-range of the voltages available, in other words, for example, at a reference voltage equal to Vdd/2 for an audio amplifier supplied between a positive supply voltage Vdd and ground. The maximum dynamic range is thus obtained for the amplification of an input audio signal. For example, the amplifier possesses an input stage having a differential structure which amplifies the difference between the input audio signal and the reference voltage.
When turning on the power to the audio amplifier, in order to avoid the production of an audible parasitic noise (referred to as “pop” in the jargon of those skilled in the art), it is known to incorporate circuitry called “antipop” circuitry. The function of this circuitry is to generate the reference voltage such that it takes the form of a rising ramp when the power is turned on. In addition, the application of the input audio signal is prevented until the end of the ramp. The ramp provides a smooth transition of the reference voltage between the zero voltage level and the bias voltage level Vdd/2 over a sufficiently long period (typically of the order of a hundred milliseconds) to avoid the generation of a pop. When the ramp has reached its final level, the application of the input audio signal is authorized. The output audio signal, corresponding to the amplified input audio signal, drives the loudspeaker of the appliance or the headphones plugged into the headphone socket of the appliance.
In general, the input stage of the amplifier includes a differential pair of MOS (Metal Oxide Semiconductor) transistors, for example p-type MOS (or pMOS) transistors. The gate of a first transistor of this pair is coupled to the input of the device to receive the input audio signal. The gate of the second transistor of this pair receives the reference voltage.
Generally speaking, the bulk of a pMOS transistor is usually biased at the positive supply voltage, or else it is connected to the transistor source. Here, this poses the following problem:
either the bulk of the pMOS transistors is biased at the positive supply voltage, so that the differential pair operates correctly for the values of the reference voltage closest to zero, in other words at the beginning of the ramp. In fact, owing to the bulk effect, the threshold voltages VT of PMOS transistors increase from about 0.7 V to about 0.9 V when their bulk-source voltage Vbs is equal to about 2 V. Their drain-source voltage Vds is therefore higher than 0.1 V, even at the beginning of the ramp. In this case, however, the PSRR (Power Supply Rejection Ratio) is degraded during the normal operation phase. Indeed, the amplifier gain is then more strongly dependent on the supply voltage Vdd.
or, alternatively, the bulk of the MOS transistors is biased at their source voltage, which will yield an optimum PSRR since the gain of the differential pair is now independent of Vdd. However, on powering up the amplifier, the latter will only operate when the drain-source voltage of the MOS transistors of the differential pair has reached about 0.1 V. Since the amplifier operates as a follower of the reference voltage during the power-up phase, the abrupt starting of the operation of the differential pair generates a voltage spike in the audio output signal. This voltage spike is heard as a pop and is therefore problematic.